There are many of different Local Area Network (LAN) technologies. Ethernet is one of the most commonly used LAN technologies. Ethernet uses hub/bus topology. In those systems using bus topology, a plurality of processor based circuit packs or nodes share a common data communication medium, and accordingly, some form of mechanism is required for controlling access to the data communication medium by each node.
The medium access control protocol used by Ethernet is Carrier Sense Multiple Access/Collision Detection (CSMA/CD), in which each node monitors the data communication medium and transmits a message when the medium is idle. If two or more nodes attempt to transmit at the same time, a collision occurs. If a collision is detected during the transmission, each node ceases transmission and re-transmit after waiting for random amount of time.
This CSMA/CD system works well while the medium is relatively quiet and there are relatively a few collisions occurring. However, when the medium becomes busy, excessive collisions reduce the overall throughput of the network. Although this system does not favour any node and all nodes can have access to the medium in theory, in worst cases, some nodes may not be able to access the medium due to excessive collisions.
In addition, Ethernet uses transformers to exchange data between nodes, which is extra circuitry not needed for intra system communication.
U.S. Pat. No. 5,479,395 issued on Dec. 26, 1995 to Goodman et al and assigned to the same assignee as the present invention discloses a serial bus system which operates in an arbitration or scheduling phase and a transmission phase. The system uses a set of four signals, an arbitration clock, a data clock, an arbitration signal and a slot signal. All nodes are connected to those four signals. In order to ensure equal access to all nodes wishing to transmit, each node is provided with an access control means which uses the arbitration signal to ensure only one node accesses the bus, and uses the slot signal to identify the point at which a node has "won" control of the bus so that the other nodes can "win" before the node which has "won". Use of four signals and the access control means add complexity in each node. Also, this system does not allow different levels of priority in transmission requests.
Accordingly, there is a need for an improved message communication system which is simple and provides reliable access for each node to the medium, and which allows use of different levels of priority and easy communication with nodes in other network systems.